In the field of air traffic monitoring, the tracking of aircraft in a given area of the air space is conventionally handled by two types of radars, the so-called primary radars whose operation is otherwise known and which require no cooperation of the targets being tracked, and the secondary radars whose operation follows an interrogation-response approach which requires the interrogated target, cooperating target, to respond according to a determined protocol. The operating time of a secondary radar is thus mainly devoted to the emission of successive interrogation messages and to the reception of the corresponding responses emitted by the various targets being tracked.
These days, air traffic control by means of secondary radars uses a large number of interrogation modes. These modes are notably characterized by the type of aircraft, civilian or military, to which they are addressed and by the general or dedicated nature of the interrogations performed. Thus, in particular, the following can be distinguished:
the IFF interrogation modes, for purely military use, such as the SSR 1, 2 and 4 modes for which the interrogations emitted by the secondary radar are addressed to all the military aircraft;
the SSR interrogation modes, for civilian and military use, such as the SSR 3/A and C modes for which the interrogations emitted by the secondary radar are addressed generally, that is to say, addressed to all the civilian and military aircraft;
the “All Call” (or “AC”) interrogation modes for which the interrogations emitted by the secondary radar are also addressed a priori to the mode S aircraft not yet managed by the radar in selective interrogations;
the “Roll Call” (or “RC”) interrogation modes, for which the selective interrogations emitted by the secondary radar are specifically addressed, each selective interrogation being addressed to a given aircraft which is the only one to respond to this interrogation.
Because of the increasing density of aircraft moving in the same area of space, and the diversity of the interrogation modes, optimizing radar time, that is to say, optimizing the number of interrogations that can be performed at a given time, in principle represents a task that is difficult to complete successfully. In practice, the time at the end of which the response to a given interrogation reaches the radar is notably linked to the protocol employed by the interrogation and to the distance from the aircraft to the radar, a distance that is situated within the limits of the instrumented range of the radar.
The problem of optimizing the time management is therefore becoming increasingly acute, both for civilian and military systems, responsible for air traffic control (ATC). To address the safety constraints, the civilian radar (ATC) in fact have to be able to both provide a long-range coverage, and, for each target being tracked, handle a large number of reply hits on the target, especially for the SSR and IFF interrogation modes, while implementing increasingly high antenna rotation speeds, relative to the rates demanded for the refreshing of the information by the radar. For their part, the military radars, in addition to the interrogation tasks devolved to the civilian radars, the SSR—3/A and SSR_C modes and mode S (or MS) in particular, also have to handle the interrogation tasks in military modes (modes 1, 2, 4, 5).
The result of this is that the illumination time that can be allocated to a given target becomes a rare resource, and all the more so when the radar rotation speed is high and the number of targets to be processed continues to increase.
One solution that is applied these days in the field of military radars consists in spreading over a number of revolutions (conventionally from two to four revolutions) the implementation of these various modes of interrogation of the aircraft being tracked. Consequently, the rate of renewal of the information becomes lower, which gives a result in terms of the refreshing of the information that is opposite to that sought by the use of a higher antenna rotation speed. Such a solution is, moreover, incompatible with the safety constraints imposed in the field of civilian air traffic control, which impose a regular renewal of the information on each antenna revolution.
In the current state of the art, the inclusion of the safety constraints imposed by the international civilian aviation authorities have hitherto led the manufacturers in the field to allocate periods dedicated to the different types of interrogation used (AC, RC, IFF or SSR) and to sequence these interrogations alternately. Each type of interrogation is thus allocated a period of defined duration so that the radar time is broken down into a series of consecutive periods, alternately dedicated to each type of interrogation. The number of IFF and SSR periods must be sufficient for each revolution of the antenna to ensure the correct detection of the aircraft in the SSR and IFF mode concerned.
The Eurocontrol document entitled “Principles of Mode S Operation and Interrogation Code” dated 18 Mar. 2003 also clearly explains that a temporal allocation for each task is recommended.
Thus, in the state of the art, during each time slot dedicated to the interrogation periods of RC (“Roll Call”) type, the secondary radar is constrained, the management of the modes being as it is, to addressing two imperatives:
to manage the interleaving of the mode S selective interrogation tasks for the aircraft operating in this mode and located within the antenna lobe concerned and of the tasks for receiving and processing the selective responses returned by these aircraft so that, in particular, the emission of an interrogation does not occur at an instant for which another selective response to a preceding selective interrogation reaches the radar, nor that two selective interrogations or even two selective responses overlap;
to selectively interrogate successfully, that is to say, generating a selective response that is positively decoded by the radar, at least one time, each aircraft operating in mode S during the time slot during which its position is scanned by the antenna lobe.
The arrangement of these tasks must also be performed without overlap so as to avoid the synchronous interference effects between selective responses.
When mode S was introduced, the proportion of aeroplanes equipped with means enabling them to use this mode S (MS) was low, around 20%. Consequently, in the fixed time slot dedicated to the Roll Call interrogations, it was possible to interrogate all the aircraft likely to be interrogated in mode S, given the width of the radar beam and its position at the instant concerned. The rest of the aircraft, in a proportion of around 80%, were then the subject of interrogations in IFF mode, in SSR mode or in All Call (or AC) mode S.
At present, the situation is greatly reversed and it can be assumed that in Europe, notably, most aircraft (more than 95%) are equipped with means enabling them to be interrogated in mode S whereas only 5% are equipped with means enabling them to be interrogated only in SSR and IFF modes. The main consequence of this reversal is that, since the durations of the interrogation periods that correspond to the various modes are fixed, the time interval dedicated to the Roll Call interrogations is becoming insufficient to address the needs of the traffic whereas, at the same time, those dedicated to the All Call or SSR interrogations are largely underused.
The principle of a fixed allocation of the duration of the time slots corresponding to each type of interrogation and the alternation of the corresponding tasks is therefore no longer appropriate and it is better to implement a more suitable principle which does, however, observe the general constraints imposed by the international civil aviation authorities.